Classification of Ice Crystal Shapes in Midlatitude Ice Clouds from Three Years of Lidar Observations over the SIRTA Observatory

Abstract

This paper presents a study of ice crystal shapes in midlatitude ice clouds inferred from a technique based on the comparison of ray-tracing simulations with lidar depolarization ratio measured at 532 nm. This technique is applied to three years of lidar depolarization ratio observations from the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory in Palaiseau, France, amounting to 322 different days of ice cloud observations. Particles in clouds are classified in three major groups: plates, columns, and irregular shapes with aspect ratios close to unity. Retrieved shapes are correlated with radiosounding observations from a close-by meteorological station: temperature, relative humidity, wind speed, and direction. Results show a strong dependence of the relative concentration of different crystal shapes to most atmospheric variables, such as the temperature, with a clear successive dominance by platelike (temperature above ?20°C), irregular (temperatures between ?60° and ?40°C), and columnlike shapes (temperatures below ?60°C). Particle shapes are almost exclusively columnlike below ?75°C. This is in sharp contrast with previous results of the same classification applied to tropical clouds, and highlights the high geographic variability of the ice clouds distribution of microphysical properties. Results also suggest that ice clouds created by jet streams (identified by high wind speeds) are strongly dominated by columnlike shapes, while front-created ice clouds (identified by lower wind speeds) show a much more variable mix of shapes, with the dominant shapes depending on other factors. Results also suggest different microphysical properties according to the average direction source of air masses and winds. Following these results, a possible parameterization of ice crystal shapes in midlatitude ice clouds as a function of temperature is provided.